The present invention relates to the construction of hybrid towers having a lower part made of concrete and an upper part made of metal.
Typically, such a tower is used to support the nacelle of an in-shore wind generator of large power (e.g. 3 MWatt or more) high above ground level (e.g. about 140 m).
When the height of a wind generator tower increases, the cost of conventional cylindrical steel masts becomes excessive. To circumvent the technological limitation of cylindrical metallic masts, some manufacturers have developed towers made of steel trellis (or truss). However, such towers have poor esthetics.
Other manufacturers make the tower with concrete cast in-place on at least part of the height of the tower, using climbing or sliding formwork. This kind of construction method is sensitive to temperature conditions which can be experienced on the site, slowing or preventing setting of the concrete, and also to wind conditions since the concrete armatures must be mounted with a tall crane. Such a construction method becomes very expensive for very high towers.
Some wind generator towers are made of prefabricated concrete elements over their whole height. The elements are lifted using a tall crane. They can be assembled into annular segments on the ground, the crane being powerful enough to raise a whole segment, or directly in their high position. A drawback of such technique is its sensitivity to wind and the associated regulatory constraints, causing a strong impact on construction delays. Another limitation is the availability of cranes of sufficient power and height, which must be reserved months in advance.
Some telescoping techniques have also been proposed to build wind power plants. For example, DE10111280 discloses a wind power plant having mast segments raised up by a lift device in order to insert new segments from below. JP 1 190883 discloses a method of lifting an iron tower on top of a high building using jacks and a balance weight secured to the bottom of the iron tower. Another lifting device for the assembly of a wind turbine is disclosed in WO 2007/125138 A1.
The concept of a hybrid tower, made of concrete on part of the height (e.g. 50 m or more) and an metal mast on top of it, makes it possible to take advantage of an industrial concrete process and to avoid the requirement, for the civil engineering contractor, to handle heavy loads using high capacity cranes. Such a technology is compatible with the organization of wind generator providers having their own special cranes to install their nacelles. However, such a technology has hardly been implemented in practice.
There is a need to further develop the hybrid tower concept to make it more practicable.
It is proposed a method of building a hybrid tower, comprising:                erecting a metal mast connected to a foundation;        building a concrete structure having a plurality of superimposed concrete segments around the metal mast by using the metal mast as a support;        disconnecting the metal mast from the foundation;        lifting the metal mast by telescoping and guiding the metal mast along the concrete structure; and        connecting the lifted metal mast to the concrete structure.        
The step of building of the concrete structure comprises:                a) installing at least one first segment of the concrete structure;        b) lifting the installed segment(s) of the concrete structure along the metal mast by guiding the lifted segments of the concrete structure along the metal mast using suitable bearing devices, so as to clear a space having a sufficient height to receive a next segment of the concrete structure;        c) installing the next segment of the concrete structure in said space; and        d) repeating steps b) and c) until a last segment of the concrete structure is installed at a bottom part of the concrete structure.        
The metal mast is used as a guiding support when building the concrete structure, and afterwards the concrete structure is used as a support to raise the metal mast. The method is well suited to the industrial installation of large windmill farms, with many steps carried out at ground level where environmental conditions are better handled or controlled.
The concrete structure is typically built by assembling prefabricated elements, although in situ casting methods may also be used for at least part of the height of the concrete structure.
In an embodiment, a guiding structure is connected to a bottom portion of the metal mast, the guiding structure having a lower portion provided with a guide part for cooperating with the concrete structure when the metal mast is lifted. The guiding structure can be removably connected to the bottom portion of the metal mast, and disconnected after the step of lifting the metal mast so as to be reusable for building another tower.
The guiding structure can be placed between the metal mast and the foundation in the step of erecting the metal mast, and used as a lifting support structure in the step of building the concrete structure.
Other features and advantages of the method and tower disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings.